Monitoring of the oxygenation level of a tissue region is required in order to determine if the tissue is viable or necrotic. For example, measuring the oxygen saturation level allows for determining whether a patient that experienced a stroke event should undergo a therapeutic procedure, whether a certain procedure is not necessary or whether performing certain procedures entails a high risk. Such measurements are also imperative for determining the efficiency of a treatment.
Near infrared light has been used to non-invasively probe the patient's brain based on different absorption characteristics of oxygenated and deoxygenated hemoglobin. However, near infrared spectroscopy (NIRS) suffers from several drawbacks associated for example with the fact that differential scattering of two different wavelengths used in the measurements result in an uncertainty in the path length that each wavelength passes; an inherent inability to localize the probed volume which requires computation-intensive tomographic devices and algorithms to resolve; analysis of the detected signal depends on a model being used to characterize the tissue structure being probed. This impedes the use of NIRS in real time medical settings.
U.S. Pat. No. 5,293,873 discloses a measuring arrangement for tissue-optical examination of a subject with visible, NIR or IR light. According to this technique, coherent light and ultrasound are directed at the subject along parallel propagation paths. The ultrasound causes a Doppler shift in the light emerging from the subject, this shift being related to certain tissue characteristics. The light emerging from the subject is detected and a corresponding signal is supplied to an evaluation stage which absolutely or relatively calculates the intensity of those parts of the detected light which proceeded through tissue not charged by ultrasound and those parts of the detected light which proceeded through tissue charged by ultrasound.